The packing efficiency in a body centered cubic $(bcc)$ structure is closest to $.... \, \%$

The ratio of close-packed atoms to tetrahedral holes in cubic close packing is

In a compound,nitrogen atoms $(N)$ make a cubic close-packed lattice and metal atoms $(M)$ occupy one-third of the tetrahedral voids present. Determine the formula of the compound formed by $M$ and $N$.

What is the percentage of void space in $hcp$ and $ccp$ structures (in $\%$)?

$A$ compound is formed by two elements $X$ and $Y$. Atoms of the element $Y$ (as anions) make $ccp$ and those of the element $X$ (as cations) occupy all the octahedral voids. What is the formula of the compound?

Fill in the blanks:
$(a)$ In $ccp$ structure,$a = \dots \dots r$.
$(b)$ In $hcp$ structure,packing efficiency $= \dots \dots \%$.
$(c)$ In $bcc$ structure,percentage of packing efficiency $= \dots \dots \%$.